It's not that a water pump can't go 200k, my sister never replaced the water pump in her Toyota and it has 180k+ on it now.
Changing out a water pump with the T-belt isn't mandatory, just preventive maintenance. More of an insurance thing or piece of mind. I've gone 2 T-belt changes before the water pump, so it had 120k on it. You aren't guaranteed a failure if you don't change it, but the odds do go up.
You don't change out the alternator with every timing belt because it has easy access. Water pumps are more labor intensive to replace on their own so it just makes economic sense to replace while you're in there.
Another factor is the potential of a bad water pump is bearing seizure which can cause the T-belt to jump or break. Not a good thing, so again, it's insurance.
I'm not saying you don't roll the dice if you don't change a water pump, but there isn't any reason to think it couldn't go the distance. Depends on if your a risk taker.
Edited by Suzam, 15 July 2009 - 08:58 AM.